1. Field of the Invention
The present invention concerns a method of inserting a message such as a secret watermark in a digital signal.
It also concerns a method of extracting a message inserted in a digital signal.
Correspondingly, the present invention concerns a device for inserting a message and a device for extracting the message, adapted respectively to implement the insertion and extraction methods according to the invention.
2. Description of the Related Art
The digital signal considered hereinafter will more particularly be a digital image signal.
The message insertion envisaged in the context of the invention lies in the technical field of watermarking digital data, which can be interpreted as the insertion of a watermark in the digital data making it possible for example to authenticate the content of a digital data file. This watermarking is also referred to as digital tattooing.
Watermarking in general terms comprises the modification of coefficients representing the digital image. This modification is imperceptible to the eye but can be decoded by an appropriate decoder.
The concern here is with the robust insertion of a message.
The insertion of a message is said to be robust if the message can subsequently be extracted even if the image has undergone geometric distortions such as dividing part of the image, a change of scale or a rotation.
In order to obtain such robustness, solutions have been proposed.
The document “Data Hiding for Video-in-Video” by M. D. Swanson, B. Zhu and A. H. Tewfik, which appeared in International Conference on Image Processing, 1997, p. 676-679, proposes a method of inserting a message in an image according to which an image is divided into blocks and then each of these blocks is transformed according to a transformation of the DCT type (from the English Discrete Cosine Transform).
The coefficients of the transformed blocks are then modified so that the projection of these blocks onto pseudo-random sequences is quantized according to one of two values, chosen according to the value of the binary symbol which it is wished to insert in the block.
This method is not robust to geometric distortions since the division into blocks cannot be reproduced after geometric distortion.
The document “Preprocessed and Postprocessed Quantization Index Modulation Methods for Digital Watermarking” by B. Chen and G. W. Wornell, which appeared in Security and Watermarking of Multimedia Content (E100), San Jose, January 2000, and the document “Provably Robust Digital Watermarking” which appeared in Proc. of SPIE: Multimedia Systems and Applications II, Vol. 3845, by the same authors, present a message insertion in an image according to which binary values are inserted by scalar or vector quantization of one or more pixels of the image.
For example, if scalar quantization is used, the pixel is modified into its quantized value by a first quantizer if the value to be inserted is zero or by a second quantizer if the value to be inserted is one.
The extraction is carried out by identification of the quantizer used for each pixel.
This method is not robust to geometric distortions. This is because, according to this method, the bits to be inserted are associated with the pixels of the image according to a predefined division. In the event of geometric distortion between insertion and extraction, it is no longer possible to find this division. Consequently the association between pixel and message bit can no longer be made and extraction becomes impossible.